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Keywords:

  • core/satellite nanostructures;
  • gold nanospheres;
  • plasmon coupling;
  • plasmon resonance;
  • surface-enhanced Raman scattering

Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Results and Discussion
  5. 3 Conclusion
  6. 4 Experimental Section
  7. Acknowledgements
  8. Supporting Information

Gold nanocrystals and nanoassemblies have attracted extensive attention for various applications, including chemical and biological sensing, solar energy harvesting, and plasmon-enhanced spectroscopies, due to their unique plasmonic properties. It is of great importance to prepare shape-controlled Au nanocrystals with high monodispersity over a large range of sizes. In this work, Au nanospheres with sizes ranging from 20 nm to 220 nm are prepared using a simple seed-mediated growth method aided with mild oxidation. As-prepared Au nanospheres are remarkably uniform in size. The resultant Au nanospheres of different sizes are ideal building blocks for constructing plasmonic nanoassemblies. Core/satellite nanostructures are assembled out of differently sized Au nanospheres with molecular linkers. The core/satellite nanostructures show a red-shifted plasmon resonance peak in comparison to that of the Au cores, which is consistent with the results calculated according to Mie theory. As predicted by finite-difference time-domain simulations, the assembled core/satellite nanostructures exhibit strongly enhance Raman signals. This facile growth of Au nanospheres and assembly of core/satellite nanostructures are expected to facilitate the design of new nanoassemblies with desired plasmonic properties and functions.

1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Results and Discussion
  5. 3 Conclusion
  6. 4 Experimental Section
  7. Acknowledgements
  8. Supporting Information

Localized surface plasmon resonances, which stem from the collective oscillations of the conduction-band electrons in noble metal nanocrystals, can be controlled by the shape, size, composition, and surrounding environment of the nanocrystals. The unique plasmonic properties of noble metal nanocrystals have enabled diverse applications in chemical/biochemical sensing,[1, 2] nanomedicine,[3-5] photocatalysis,[6, 7] and solar energy harvesting.[8, 9] Moreover, the plasmon coupling between closely placed metal nanocrystals brings about more fascinating phenomena, such as the excitation of new plasmon modes,[10, 11] plasmonic Fano resonances,[12, 13] and dramatic electric field enhancements in the gap regions.[14] Gold nanospheres (NSs) are often employed in plasmon-coupled systems owing to their perfect geometrical symmetry, well-developed and simple analytical method for calculating the electromagnetic responses of them and their nanoassemblies, and their easy preparation by wet-chemistry approaches. The electrodynamic response of particles in spherical shapes can be analytically and time-efficiently solved with Mie theory in a classical electromagnetic framework.[15] The isotropic geometry of NSs facilitates the studies of the effect of the configurational symmetry breaking on plasmon coupling in nanocrystal clusters.[11, 13] Furthermore, Au NSs can function as ideal models in the investigation of quantum tunneling and nonlocal effect.[16, 17]

There have recently been a few works on the preparation of quasi-spherical colloidal Au nanocrystals with sizes controllable in a relatively large range.[18-21] Preparation of quasi-spherical Au nanocrystals in nano- to micro-scale sizes using a multi-step seed-mediated approach with the assistance of iodide ions has also been realized.[22] Nevertheless, challenges still remain in simplifying the preparation procedures, removing byproducts of irregular shapes, and/or reducing the surface roughness. The rough surface of Au nanocrystals severely affects their far-field and near-field plasmonic properties.[23, 24] The preparation of Au nanocrystals with nearly spherical shapes and smooth surfaces, therefore, is still in high demand. In addition, Au NSs of diameters variable over a wide range can serve as ideal building blocks for constructing plasmonic nanoassemblies,[11, 13, 25-28] among which core/satellite nanostructures have recently aroused intense interest owing to the presence of a large number of hot spots in the gaps between the core and satellites.[26-28] As a result, core/satellite assemblies made of metal NSs possess great potentials in surface-enhanced Raman scattering (SERS) applications, since the SERS enhancement factor is evaluated as the product of the local electric field intensity enhancements at both the excitation and Raman scattering frequencies.[29] SERS-based monitoring of chemical reactions catalyzed by Au satellites can be realized.[28] Moreover, molecularly-driven plasmonic switches have been demonstrated by controlling the inter­particle separation in reconfigurable DNA-linked core/satellite nanoassemblies.[30] Therefore, convenient and versatile methods for realizing strongly plasmon-coupled core/satellite nanostructures with small gap distances are highly desirable for further advancing their practical applications.

Herein we present the growth of Au NSs with diameters ranging from 20 nm to a few hundred nanometers, using a simple seed-mediated growth method assisted with mild oxidation. The obtained Au NSs are remarkably uniform in sizes and have smooth surfaces. They are thereafter employed for the construction of core/satellite nanostructures, with large NSs as the cores and small NSs as the satellites, through simple assembly with molecular linkers. The scattering characteristics of the core/satellite nanostructures are investigated experimentally with dark-field scattering spectroscopy and theoretically with Mie theory. The SERS activities of the core/satellite nanostructures are also examined.

2 Results and Discussion

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Results and Discussion
  5. 3 Conclusion
  6. 4 Experimental Section
  7. Acknowledgements
  8. Supporting Information

2.1 Gold Nanospheres

We prepared small Au NSs following a seed-mediated growth procedure that was modified from previously reported works.[31, 32] Large Au NSs were produced through overgrowth of the small Au NSs in conjunction with mild oxidation. Figure 1a shows the transmission electron microscopy (TEM) image of the obtained small Au NS sample. The small NSs have a nearly spherical geometry and an average diameter of 24 nm. In order to prepare larger Au NSs, we first employed the small Au NSs as seeds and grew them into Au nanocrystals of larger sizes by use of different amounts of the small Au NS seed solution. The resultant Au nanocrystals, although larger, display high-index facets and are not round in shape. The scanning electron microscopy (SEM) images of the two representative Au nanocrystal samples in different sizes are shown in Figure S1 in the Supporting Information. The obtained Au nanocrystal samples are polyhedrons with relatively sharp vertices and edges. Previous works have shown that mild oxidation can be utilized to reshape Au nanocrystals into desired shapes and sizes, where etching occurs preferentially at surface sites with high curvatures.[33-35] We therefore transformed the Au nanopolyhedrons into rounded NSs by a mild oxidation process in the presence of HAuCl4 and cetyltrimethylammonium bromide (CTAB). Figure 1b−l and Figure S2 in the Supporting Information show the TEM and SEM images of the Au NS samples with increasing diameters obtained by oxidizing the Au nanopolyhedron samples grown from decreasing amounts of the small Au NS seed solution. The as-prepared Au NS samples possess a spherical shape, smooth surfaces, and narrow size distributions (Figure S3, Supporting Information) with relative standard deviations of 4%−8%. For comparison, the relative standard deviations of the oxidation products in previous studies[33-35] are in the range of 11%−15%. The smaller relative standard deviations of our Au NS samples are ascribed to the uniformity of the Au nanopolyhedron samples on which oxidation was performed. The produced Au NS samples tend to form hexagonally packed superstructures during the preparation of the TEM and SEM samples as a result of their high shape and size monodispersity.[36]

image

Figure 1. TEM (top row) and SEM (bottom row) images of the obtained Au NS samples with increasing diameters. (a) 24 ± 2 nm. (b) 54 ± 2 nm. (c) 88 ± 4 nm. (d) 137 ± 9 nm. (e) 177 ± 14 nm. (f) 221 ± 16 nm. (g) 54 ± 2 nm. (h) 69 ± 3 nm. (i) 88 ± 4 nm. (j) 137 ± 9 nm. (k) 177 ± 14 nm. (l) 221 ± 16 nm.

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We found that the size of the as-grown Au nanopolyhedrons, and therefore the size of the final Au NSs, are dependent on the added amount of the small Au NS seeds when the same growth and oxidation conditions were applied. For the Au NS samples with diameters of 54 ± 2 nm, 69 ± 3 nm, 88 ± 4 nm, 103 ± 6 nm, 137 ± 9 nm, 177 ± 14 nm, and 221 ± 16 nm, the injected volumes of the seed solution were 4 mL, 2 mL, 1 mL, 0.5 mL, 0.25 mL, 0.125 mL, and 0.0625 mL, respectively. The volume of the growth solution was kept unchanged and the same amount of HAuCl4 was used to oxidize the Au polyhedron samples. As shown in Figure 2a, the cubic power of the diameter of the NSs (D3) is linearly dependent on the reciprocal of the volume of the seed solution (V−1) with a high determination of coefficient of R2 = 0.999. Such a simple and precise relationship makes it convenient to calculate the amounts of the seeds needed to produce Au NS samples with desired sizes within a relatively large range. A similar relationship has been obtained before in a seeded-growth method.[21] This can be easily understood, since the amounts of the growth solution and the oxidizing agent, HAuCl4, are fixed. The linear relationship also suggests that the growth of gold occurs exclusively on the seed nanocrystals without the formation of new nuclei. The more seeds added, the less amounts of Au atoms are grown onto the individual seed nanocrystals. The size of the finally obtained Au NSs is therefore determined by the amount and diameter (Dseed) of the seeds according to the formula below:

  • display math(1)

where C is a constant for the procedure using fixed amounts of the growth solution and the oxidation agent. According to this equation, the linear fitting gives the slope C and the cubic seed diameter. In the fitting plotted in Figure 2a, only the average diameters of the NS samples are employed. If the standard deviations in the measured diameters are taken into account, the slope and cubic seed diameter are obtained to be (6.3 ± 0.6) × 105 nm3 mL and (0.4 ± 2.8) × 104 nm3, respectively. The experimentally measured cubic seed diameter value is (1.4 ± 0.3) × 104 nm3. The experimental value falls in the fitted range, confirming the validity of the equation. The relatively large standard deviation for the fitted cubic seed dia­meter results from the considerably large standard deviations of the experimentally measured diameters, which are in the range of 2.2 × 104 nm3 to 2.3 × 106 nm3 for the overgrown Au NS samples. On the other hand, the size of the Au NSs can also be nearly continuously tailored by varying the amounts of the growth solution or the oxidation agent. The amount of the oxidation agent used in our experiments is sufficient for reshaping the nanopolyhedrons into NSs, and is not in excess in order to avoid the waste of gold. By adjusting the seed amount added in the growth solution, we can readily produce Au NSs with dia­meters ranging from 50 nm to 220 nm. As a result, our preparation method can give rise to Au NSs ranging from 20 nm to 220 nm in diameter with narrow size distributions. Au NSs larger than 220 nm can be prepared by further reducing the seed amount. However, we found that new nuclei can form in the solution due to the small amount of the Au nanocrystal seeds, which therefore causes the obtained Au NSs to have wide size distribution.

image

Figure 2. Size dependence and crystalline structure of the Au NSs. (a) Relationship between the average diameter of the Au NS sample and the added volume of the small Au NS seed solution. The error bar height for each point represents one standard deviation. (b) Electron diffraction pattern recorded along the [110] axis of a single-crystalline Au NS. The inset is the low-magnification TEM image of the NS on which the diffraction pattern was taken. (c) High-resolution TEM image of a twinned Au NS. The inset is the low-magnification TEM image of the NS on which the main image was recorded.

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We carefully characterized the crystalline structure of the resultant Au NSs. Both single-crystalline and twinned Au NSs were found in the same batch of products. Figure 2b shows the electron diffraction pattern of a [110]-oriented Au NS from the sample with an average diameter of 69 ± 3 nm. The diffraction pattern is consistent with the face-centered cubic structure of single-crystalline gold. In contrast with the single-crystalline Au nanocrystals, some Au NSs from the same TEM sample display clear dividing lines across the spheres (Figure 2c, inset), which are believed to be twin boundaries. The twin boundaries are confirmed by the high-resolution TEM images taken at the dividing lines (Figure 2c). In fact, the small Au NSs used as the seeds inherently contain a mixture of single-crystalline and twinned nanocrystals (Figure 1a). 61 out of 141 small Au NSs, counted from the TEM images, were found to be twinned nanocrystals. Although seed-mediated growth employing CTAB surfactant and CTAB-capped seeds generally gives single-crystalline Au nanocrystals, the generation of twinned Au nanocrystals has also been reported.[37, 38] In our prior work, we have observed that the crystalline structure of Au nanocrystal cores plays an important role in the growth of bimetallic nanostructures.[39] Under the growth conditions we employed in this work, both single-crystalline and twinned Au nanocrystal seeds can grow into larger nanopolyhedrons and can subsequently be rounded into NSs.

The resultant colloidal Au NS samples are stable in water and can keep their spherical shape for a long period of time (>∼6 months). Precipitation occurs for the Au NSs of large sizes after a certain period of storage time. Hand-shaking or gentle sonication can readily redisperse them into solutions without the formation of aggregates. Figure 3a shows the digital photograph of the colloidal solutions of the obtained Au NSs with different sizes. The Au NS samples exhibit distinct colors ranging from red to purple and yellow. Their extinction spectra (Figure 3b) were recorded on a UV/visible/NIR spectrophotometer. The dipolar plasmon mode red-shifts gradually from 521 nm to 806 nm, when the average diameter of the Au NSs increases from 24 nm to 221 nm. Peak broadening is clearly observed owing to increasing radiative losses for the Au NSs with larger sizes.[40] For the Au NSs with average diameters of 177 nm and 221 nm, quadrupolar plasmon modes at 547 nm and 565 nm, respectively, also appear. The experimental extinction spectra are seen to be in good agreement with the ones calculated from Mie theory (Figure 3c). Small discrepancies between the measured and calculated extinction spectra are believed to stem from the deviation of the prepared Au NSs from an ideally spherical shape and their small size distributions. In the calculations from Mie theory, the Au NSs are modeled as perfect spheres with their diameters set to be the average values of the experimentally measured ones.

image

Figure 3. Colors and extinction spectra of the Au NS samples. (a) Digital photograph of the colloidal solutions of the Au NS samples with different sizes, which from left to right correspond to the samples 1 to 8, respectively. (b) Normalized extinction spectra of the samples 1 to 8 mea­sured on a UV/visible/NIR spectrophotometer. (c) Normalized extinction spectra calculated according to Mie theory. The samples 1 to 8 are the Au NS samples with average diameters of 24 ± 2 nm, 54 ± 2 nm, 69 ± 3 nm, 88 ± 4 nm, 103 ± 6 nm, 137 ± 9 nm, 177 ± 14 nm, and 221 ± 16 nm, respectively. In the calculations, the Au NSs are modeled with diameters being equal to the corresponding average values. All of the extinction spectra are normalized for the purpose of comparison.

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2.2 Core/Satellite Nanostructures

To demonstrate the potential of the resultant Au NSs for constructing interesting plasmonic superstructures, we assembled them into core/satellite nanostructures. The Au NSs with an average diameter of 180 ± 14 nm were utilized as the cores, while the small Au NSs with an average diameter of 24 ± 2 nm were used as the satellites. The large size ratio between the cores and satellites was deliberately chosen to promote their assembly. The dependence of assembly on the core/satellite size ratio arises from the steric constraints imposed by the size and shape of the NS core and satellites. If the size ratio is increased, there will be more space for the adsorption of satellites. Moreover, the reduced curvature brought by increased core sizes can facilitate the attachment of satellites through bonded molecular linkers. 180-nm Au NSs were first deposited on indium tin oxide (ITO)-coated glass slides or silicon wafers, followed by the functionalization with 4-aminothiophenol (ATP) molecules. The thiol group on one end of ATP is bonded to the surface of the Au NSs to give a self-assembled monolayer of the molecular linkers. The amino group on the other end of ATP can capture the small 24-nm Au NSs, when the substrates carrying the 180-nm Au NSs are immersed in the colloidal solutions of the small Au NSs, through the formation of noncovalent linking.[26] The assembly details can be found in the Experimental Section. Figure 4 shows the low- and high-magnification SEM images of the as-assembled core/satellite nanostructures on an ITO substrate and a silicon wafer. The successful adsorption of the satellite Au NSs on the surface of the core NSs can be clearly observed. The satellite NSs are randomly distributed on the surface of the cores, both on the top and at the side. Some small Au NSs are also seen to be deposited on the flat surface of the ITO substrate. In contrast, the small NSs are rarely seen on the flat surface of the silicon substrate. These results suggest that the surface characteristics of substrates are also crucial in the exclusive fabrication of plasmonic superstructure arrays. 1,8-Octanedithiol (ODT) molecules, with two thiol groups on both ends for bonding to the core and satellite NSs, can also be employed as the molecular linkers. The core/satellite nanostructures assembled with ODT look similar to those made with ATP (Figure S4, Supporting Information). In addition, the small Au NSs are also observed to deposit on ITO instead of Si substrates.

image

Figure 4. SEM images of the core/satellite nanostructures. (a) On an ITO substrate and at low magnification. (b) On the ITO substrate and at high magnification. (c) On a Si substrate and at low magnification. (d) On the Si substrate and at high magnification. The nanostructures were prepared with ATP as the molecular linker.

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Our previous study has shown that Si substrates can alter the plasmonic properties of deposited Au nanocrystals greatly while ITO substrates only induce a weak scattering peak in the spectral region between 500 nm and 550 nm.[41] We therefore performed the scattering measurements of the Au NSs and the prepared core/satellite nanostructures on ITO substrates to minimize the substrate effect. The scattering signals were collected from areas of ∼0.02 mm2 on the substrate surface using a 5× dark-field objective (numerical aperture = 0.15). Considering that the large Au core NSs and core/satellite nanostructures are deposited at a surface number density of ∼2 cores μm−2 (Figure 4a and Figure S4a, Supporting Information), we estimate that ∼4 × 104 particles of interest contribute to the collected scattering signals. As shown in Figure 5a, the plasmon resonance wavelength undergoes a red shift from 648 nm to 723 nm after the attachment of the satellite Au NSs onto the core NSs with ATP molecules. Similarly, the scattering peak red-shifts to 716 nm for the ODT-assembled core/satellite nanostructures. The red shifts are attributed to the plasmon coupling between the Au core NS and satellite NSs. In previous studies, the perturbation of a small Au nanocrystal attached to a relatively large Au nanocrystal has proved to be affected by the sizes of both the small and large nanocrystals, the location of the small nanocrystal on the large one, and the gap distance between the two nanocrystals.[42, 43] The average numbers of the visible satellite NSs are determined from the SEM images to be 18 ± 2 and 24 ± 2 for the ATP- and ODT-assembled core/satellite nanostructures, respectively. There should be some satellites hidden at the bottom of the Au cores. Since they cannot be observed under SEM imaging, they are not counted. The difference in the numbers of the attached satellites can be ascribed to the differences in the affinity to the gold surface, the length, the rigidity, and the spatial configuration between the employed ATP and ODT molecular linkers. The difference in the numbers of the attached satellites and the gap distances, the latter of which are determined by the lengths and orientations of the molecular linkers,[27, 44] are responsible for the slight scattering spectral discrepancy between the ATP- and ODT-assembled core/satellite nanostructures.

image

Figure 5. Scattering spectra of the core/satellite nanostructures. (a) Normalized dark-field scattering spectra of the Au core NSs and core/satellite nanostructures deposited on ITO glass substrates. (b) Calculated scattering spectra of the core/satellite nanostructures with 20 satellite NSs. Five simulations were carried out, with the satellite NSs placed in different random positions. (c) Calculated scattering spectra of the core/satellite nanostructures with the number of the satellite NSs to be 0, 10, 20, 30, and 50.

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Generalized multiparticle Mie (GMM) calculation, which is an extension of Mie theory for light absorption and scattering by multiple spheres,[45] was carried out to understand the plasmonic properties of the Au core NSs when perturbed by the small satellite NSs. The calculations were performed in a homogeneous medium with an effective refractive index of 1.2 to account for the asymmetric environment of the particles on ITO substrates. Such an approximation has been verified to be valid and made GMM practicable for cases of multi­ple spheres embedded in complicated environments.[46] Au NSs with a diameter of 24 nm are randomly distributed on the surface of a Au core NS with a diameter of 180 nm at a fixed gap distance of 1 nm, which is taken according to the reported gap distances for ATP- and ODT- linked nanostructures.[26, 27, 43] The random positions of the satellites were determined with a self-coded program based on the Monte Carlo method. For the core/satellite nanostructures with 20 satellites, five sets of the satellite positions on the core NS surface were generated. The calculated scattering spectra (Figure 5b) are nearly identical, suggesting that the effect of the spatial positions of the satellites on the scattering spectra is negligible when the number of the satellites is large. Figure S5 in the Supporting Information displays the cases for the core/satellite nanostructures with 10, 30, and 50 satellites. The differences among the scattering spectra of the core/satellite nanostructures with the same number of satellites but different random satellite positions are most prominent for the case with 10 satellites, yet they are still small. We therefore excluded the effect of the satellite arrangement, and found from the calculations that when the satellite number is gradually increased from 0 to 50, the scattering peak red-shifts gradually (Figure 5c). For comparison, in our experiments, the ATP-linked core/satellite nanostructures with 18 satellites exhibit a larger red shift than the ODT-linked nanostructures with 24 satellites. This discrepancy between the experiments and calculations can be ascribed to the fact that the gap distance brought by ATP molecules with a shorter chain is slightly smaller than that caused by ODT molecules with a longer chain.

2.3 SERS

We examined the local electric field intensity enhancement of the core/satellite nanostructures with finite-difference time-domain (FDTD) simulations, because local electric field enhancements play an essential role when metal nanostructures are utilized for plasmon-enhanced spectroscopy applications. For the purpose of demonstration, the core/satellite nanostructure made of a Au NS core of 180 nm in diameter and 8 Au NS satellites of 24 nm in diameter was considered in the FDTD simulations. The gap distances between the core and satellites were set to be 1 nm, and the excitation wavelength was 633 nm. Stronger electric field enhancements are observed in the gap regions between the core and satellites in comparison to those on the surface of the sole Au NS core (Figure 6a,b). The gaps with their inter-particle axes aligned parallel to the excitation polarization direction exhibit the strongest electric field intensity enhancements, which are ∼1200 times higher than those on the surface of the sole Au NS core. The field intensity enhancement on the core/satellite nanostructure gets smaller as the inter-particle axis between the core and satellite is deviated away from the excitation polarization direction. Previous studies have shown that SERS signals are contributed dominantly by hot spots, which are mainly located in the gap regions of metal nanostructures and have very high electric field enhancements.[47] In addition, local electric field enhancements in the gap regions of metal nanostructures have also been found to be highly dependent on the excitation polarization direction relative to the inter-particle axis.[48] On the basis of these previous findings, we reason that the large electric field enhancements in the gap regions between the core and satellites and the presence of multiple satellites that are distributed randomly on the core surface make our core/satellite nanostructures ideal candidates for SERS applications. The random distribution of the satellites ensures that some hot spots will be excited under any excitation polarization direction.

image

Figure 6. Local electric field intensity enhancements and SERS spectra of the core/satellite nanostructures. (a) Electric field intensity enhancement contour around the Au NS core. (b) Electric field intensity enhancement contour around the core/satellite nanostructure. The field intensity enhancement contours are drawn on the logarithmic scale. (c) Raman scattering spectra of a bare ITO substrate, the Au NS core attached with ATP molecules, and the ATP-assembled core/satellite nanostructures. The metal nanostructures were deposited on similar ITO substrates.

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ATP is inherently an excellent SERS probe because of its large Raman scattering cross section.[26] The ATP-assembled core/satellite nanostructures were therefore directly employed to characterize the SERS performance of the nanostructures, without the requirement of further functionalization with Raman-active molecules. A 633-nm laser was focused to a spot of ∼7 μm in diameter on the surface of ITO substrates deposited with the core/satellite nanostructures or bare Au NS cores. According to the excitation spot size and the surface number density of the nanostructures, the Raman scattering signals are estimated to be collected from ∼80 nanostructures. The ATP-assembled core/satellite nanostructures emit strong Raman signals, while no discernible Raman bands are observed for the ATP-functionalized Au NS cores (Figure 6c). The two strong Raman bands at 1074 and 1577 cm−1, which correspond to the C−S and C−C stretching modes, respectively, are generally acknowledged as being strengthened through the electromagnetic enhancement mechanism. The enhancement mechanisms of the modes at 1139, 1388, and 1433 cm−1 have remained controversial. In different studies, these modes are either ascribed to the bending of the C−H bonds and the stretching of the C−C bonds in ATP, with the Raman signals enhanced jointly through the electromagnetic mechanism and a charge transfer process, or attributed to the formation of 4,4′-dimercaptoazobenzene, with the Raman signals enhanced solely electromagnetically.[49-51] During the Raman measurements, we purposely avoided sampling areas that contained the aggregates of the nanostructures by adjusting the position of the excitation laser spot (Figure S6a−f, Supporting Information). Raman spectra collected from different positions on the ITO substrate deposited with the core/satellite nanostructures exhibit good reproducibility (Figure S6g, Supporting Information). The SERS enhancement factor of the ATP molecules residing in the hot spots of the core/satellite nanostructures is estimated to be (4.0 ± 1.0) × 107, which is about four orders larger than that of the ATP molecules adsorbed on the bare NS cores (see the Supporting Information for the estimation details). The large SERS enhancement factor of the core/satellite nanostructures and the richness of hot spots brought by the large core/satellite size ratio make our SERS substrates robust and stable for functioning as Raman probes.

3 Conclusion

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Results and Discussion
  5. 3 Conclusion
  6. 4 Experimental Section
  7. Acknowledgements
  8. Supporting Information

In summary, Au NSs with good shape and size uniformity and a large range of diameters have been prepared by a wet-chemistry seed-mediated growth method in conjunction with a mild oxidation process. The average diameter of the resultant Au NSs can be finely controlled by varying the seed amount according to an established relationship between the NS diameter and the volume of the seed solution. The prepared Au NSs are mixtures of single-crystalline and twinned nanocrystals. We have demonstrated the assembly of core/satellite nanostructures out of a large Au NS sample of 180 nm in diameter and a small NS sample of 24 nm in diameter using molecular linkers. Compared with the bare Au NS cores, the core/satellite nanostructures exhibit a red shift of ∼70 nm in the plasmon resonance peak on the measured scattering spectra. The measured red shifts are in agreement with the results obtained from the GMM calculations. The FDTD simulations reveal the existence of strong local electric field enhancements in the gaps between the core and satellites. The ATP-assembled core/satellite nanostructures are demonstrated to exhibit strongly enhanced Raman signals, while no Raman bands can be distinguished for the bare Au NS cores functionalized with ATP. Our assembly approach can be extended to the assembly of nanocrystals with different compositions and shapes into heterogeneous core/satellite nanostructures. We believe that the combination of the facile growth of nearly monodisperse Au nanospheres over a broad size range and the assembly of Au NSs into core/satellite nanostructures will facilitate the design and experimental construction of unique metal nanocrystal assemblies with desired properties for various plasmon-based applications.

4 Experimental Section

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Results and Discussion
  5. 3 Conclusion
  6. 4 Experimental Section
  7. Acknowledgements
  8. Supporting Information

Chemicals: Gold chloride trihydrate (HAuCl4·3H2O, 99%), sodium borohydride (NaBH4, 98%), ascorbic acid (99%), cetyltrimethylammonium chloride (CTAC) in water (25 wt%), and ODT (≥97%) were purchased from Sigma-Aldrich. CTAB (98%) was received from Alfa Aesar. ATP (96%) was obtained from Aladdin Reagent Co., Ltd. Acetonitrile (HPLC grade) was purchased from RCI Labscan. Deionized water was utilized throughout the preparations of all the Au NS samples.

Growth of the Small Au NSs: The small Au NSs were obtained by a seed-mediated growth method. Briefly, a HAuCl4 solution (0.01 M, 0.25 mL) was first mixed with a CTAB solution (0.1 M, 9.75 mL), followed by the rapid injection of a freshly-prepared, ice-cold NaBH4 solution (0.01 M, 0.60 mL) under vigorous stirring. The resultant solution was kept under gentle stirring for 3 h at room temperature. 0.12 mL of the as-prepared seed solution was injected into a growth solution made of CTAB (0.1 M, 9.75 mL), water (190 mL), HAuCl4 (0.01 M, 4 mL), and ascorbic acid (0.1 M, 15 mL). The reaction mixture was gently shaken and then left undisturbed overnight at room temperature. The resultant small Au NS sample was washed and concentrated by four times into water by centrifugation and redispersion for further use.

Growth of the Large Au Nanopolyhedrons: The Au nanopolyhedrons were grown by the seed-mediated method using the small Au NSs as seeds. Typically, a varying volume of the seed solution (0.125−4 mL) was first added into a CTAC solution (0.025 M, 30 mL). For the seed solution at volumes less than 0.2 mL, the seed solution was diluted four times with water before use. After the sequential addition of ascorbic acid (0.1 M, 0.75 mL) and HAuCl4 (0.01 M, 1.5 mL), the mixture solution was placed in an air-bath shaker (45 °C, 160 revolutions per minute) and kept for 3 h. The obtained Au nanopolyhedrons were centrifuged and redispersed in a CTAB solution (0.02 M, 30 mL).

Preparation of the Large Au NSs: The large Au NSs were produced by oxidizing the obtained Au nanopolyhedrons with HAuCl4. The Au nanopolyhedrons in CTAB solutions were mixed with a HAuCl4 solution (0.01 M, 0.2 mL). The resultant mixture solution was kept in the air-bath shaker (45 °C, 160 revolutions per minute) for 2 h. Au NSs were produced through the mild oxidation of the Au nanopolyhedrons in the presence of HAuCl4 and CTAB. The obtained Au NSs were centrifuged and redispersed in water for storage.

Preparation of the Core/Satellite Nanostructures: The Au NSs with an average diameter of 180 nm were centrifuged one more time and used as the cores. The particle concentration was estimated to be ∼9 pM. The NSs with an average diameter of 24 nm were utilized as the satellites. They were dispersed in a water/acetonitrile mixture (1:3 v/v). The particle concentration was estimated to be ∼0.8 nM. Water/acetonitrile mixture solutions at proper compositions have been found to be able to destroy the CTAB bilayer on the Au nanocrystal surface and maintain only a CTAB monolayer so that the mercapto or amino groups on small molecules can readily penetrate through the CTAB monolayer and bond to the Au nanocrystal surface.[48, 52] ITO-coated glass slides (Shenzhen Nanbo Display Technology Co., Ltd., STN-SI-10, 20 × 8 mm2) were cleaned under ultrasonication (Blackstone-NEY Ultrasonics, 28 H) in acetone for 30 min, followed by treatment in a plasma cleaner (Harrick Scientific, PDC-32G, 18 W) for 10 min. The ITO substrates were then immersed in the large Au NS solution for 2 h. After this process, the large Au NSs were adsorbed on the ITO substrates at a high surface number density, with the substrates exhibiting a visible red color. For the adsorption of the small Au NSs, the ITO substrates were subsequently transferred into an acetonitrile solution of ATP or ODT (0.1 mM). After 1 h, they were taken out and rinsed with acetonitrile. The adsorption of the small Au NSs was realized by immersing the ITO substrates in the small NS solution for 1 h, followed by rinsing with acetonitrile and blowing dry with nitrogen. The small Au NSs were consequently adsorbed on the large NSs under the assistance of ATP or ODT molecules. During the preparation of SERS substrates, the ITO substrates deposited with the large Au NSs and functionalized with ATP were cut into two equally-sized pieces. One piece was immersed in the small NS solution, and the other was immersed in a water/acetonitrile mixture solution (1:3 v/v) without the small NSs as a control. Similar core/satellite nanostructures were formed by replacing the ITO substrates with silicon wafers. The number of satellites per core was found to increase with the immersion time of the substrates in the small NS solution and the particle concentration of the satellite solution. The immersion time of 1 h is long enough for obtaining a nearly saturated number of satellites on the Au NS cores.

Characterization: The extinction spectra were taken on a Hitachi U-3501 UV/visible/NIR spectrophotometer. The particle concentrations of the Au NSs were estimated from the peak extinction values and the extinction coefficients obtained from the GMM calculations. SEM imaging was performed on an FEI Quanta 400 FEG microscope. TEM imaging and electron diffraction were carried out on an FEI Tecnai Spirit microscope, which was operated at 120 kV. High-resolution TEM imaging was performed on an FEI Tecnai 20 ST microscope operated at 200 kV. The diameters of the Au NSs were measured on their TEM images, with ∼200 particles measured per sample. The scattering spectra were collected on a dark-field optical microscope (Olympus BX60) that was integrated with a 100-W quartz-tungsten-halogen lamp, a monochromator (Acton SpectraPro, 2300i), and a charge-coupled device camera (Princeton Instruments, Pixis 512B). The camera was thermoelectrically cooled to −70 °C during the measurements. A dark-field objective (5×, numerical aperture = 0.15) was used for both illuminating the nanostructures with white light for excitation and collecting the scattered light. The scattering spectra were calibrated with the response curve of the entire optical system. The Raman spectra were recorded on a micro-Raman system (Renishaw, RM 3000), with a HeNe laser (633 nm, 25 mW) as the excitation source and a 50× objective. The exposure time was set at 10 s.

GMM Calculations and FDTD Simulations: The GMM calculations were performed using GMM solution (Fortran code gmm01f.f, provided by Y.-L. Xu).[45] The refractive index of the surrounding water was set as 1.33. The FDTD simulations were performed using FDTD Solutions 8.0, which was developed by Lumerical Solutions, Inc. During the FDTD simulations, an electromagnetic pulse with a central wavelength of 633 nm was launched into a box containing a target nanostructure. The Au nanostructure and its surrounding space were divided into 1-nm meshes. Finer meshes of 0.125 nm in size were added at the gaps between the core and satellites. The refractive index of the surrounding environment was taken to be 1.2 to account for the substrate effect. The gap distance between the core and satellites was set as 1 nm. For all simulations, the dielectric function of gold was represented with the fitting from Johnson and Christy's data.[53]

Acknowledgements

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Results and Discussion
  5. 3 Conclusion
  6. 4 Experimental Section
  7. Acknowledgements
  8. Supporting Information

J.F.W. acknowledges the support from Hong Kong RGC (GRF, Ref. No. CUHK401511, Project Code 2130270 and CRF, Ref. No. CUHK1/CRF/12G, Project Code 2390064). H.K.W. acknowledges the support from Hong Kong RGC (GRF, Ref. No. 605210) and Hong Kong UGC (Ref. No. RPC11SC19). We thank Mr. Man Hau Yeung for his help in the high-resolution TEM measurements. The FDTD simulations were conducted in the High Performance Cluster Computing Centre, Hong Kong Baptist University, which is supported by Hong Kong RGC and Hong Kong Baptist University.

Supporting Information

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Results and Discussion
  5. 3 Conclusion
  6. 4 Experimental Section
  7. Acknowledgements
  8. Supporting Information

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